The present invention relates generally to composite material manufacture and, more particularly, to a method and apparatus for forming composite strands from glass fibers and other materials such as organic fibers.
A variety of methods exist for producing composite strands containing glass and organic material. Methods such as solution processing, slurry processing, and melt impregnation involve passing a finished glass fiber tow through a polymer-containing liquid. The polymer clings to the tow and results in a composite material. Another method, known as film stacking, requires stacking fiber tows between sheets of thermoplastic material and applying pressure for a prolonged period of time. Yet another method, called dry powder impregnation, involves applying a thermoplastic powder to a fiber tow and then heating to sinter the powder particles to the fibers.
All of the above-mentioned methods have various disadvantages. A common disadvantage of all the methods is the fact that one or more off-line (glass tow spreading) processing steps are required to produce a final composite strand product. Such off-line steps increase the cost of the product and complexity of production.
Another method for producing composite strands, fiber commingling, can be performed using an in-line process. Processes such as the one disclosed in U.S. Pat. No. 5,011,523 supply molten glass to a bushing, draw glass fibers from the bushing, and apply a size to the fibers via an applicator roller. The sized fibers are gathered into one or several yarns which are then combined with polymer fibers. The resulting mass of fibers is collected into a single tow at an assembly device and then either wound on a collet to produce a composite strand package or immediately cut into sections.
Despite being an in-line process, methods such as the one described above have several disadvantages. Of particular interest is the problem of the location of polymer within the finished strand. Specifically, the strand of composite material produced does not have thermoplastic material distributed throughout the product cross section. Rather, the polymer merely encompasses the glass fibers. Also, the amount of binder/size in the final product is very small (about 0.5%). Therefore, a composite material formed from these products will not have polymer distributed throughout its cross section and the fibers will not be strongly bound together.
Accordingly, there is a need for an improved composite strand forming process which results in dispersing thermoplastic fibers with glass fibers throughout the composite strand cross section and a relatively high binder/size percentage in the composite strand product. There is further a need for an improved process for forming a composite strand product which allows it to be produced in a more efficient and economical manner.